1. Field of the Invention
This invention relates to image forming apparatuses.
2. Description of the Related Art
In an image forming apparatus that forms an image by electronic photography process, such as printer, copier, facsimile, and multi-function peripheral thereof, for instance, a toner image is developed on a photoconductor drum, and the toner image is transferred onto an intermediate transfer belt, and then transferred from the intermediate transfer belt to a sheet of print paper, and finally the toner image is fixed on the sheet of print paper.
In such image forming apparatus, when necessary or periodically, toner density and its gradation are adjusted. In a four-color image forming apparatus, toner density and its gradation are adjusted for each of four colors.
To print a scanned document, by using a threshold conversion process (e.g. an error diffusion process or a screen process) chosen according to the type of the document, some image forming apparatuses generate data of an image to be printed. Otherwise, for a single page, a plurality of threshold conversion processes may be used. Therefore, gradation adjustment must be performed for the error diffusion process and the screen process separately.
In the case that the apparatus performs automatic gradation adjustment of the error diffusion process, for instance, the apparatus has a ROM (Read Only Memory) that stores data of a pattern image generated by a single-threshold conversion from plain patch images corresponding to grades in a gradation, and forms a toner pattern image based on the data, detects the pattern image by a sensor, and then performs gradation adjustment based on the detection by the sensor. Alternatively, the apparatus performs the error diffusion process for each of pixels sequentially to generate data of a pattern image of single-threshold conversion, and stores the generated data in a RAM (Random Access Memory); and then forms a toner pattern image based on the data, detects the pattern image by a sensor, and then performs gradation adjustment based on the detection by the sensor.